Indirect evidence strongly suggests the following sequence of events in human or rabbit cells stimulated to produce interferon with poly I. poly C: stimulation of interferon mRNA transcription occurs rapidly and is virtually complete within 1.5 - 2 hr after the exposure of cells to the inducer. This newly synthesized mRNA is being actively translated until about 3 hr after the initial induction but further translation then ceases rapidly as a result of the action of a newly synthesized cellular repressor. This repressor is thought to be a protein which prevents the translation and promotes the degradation of interferon mRNA. Further efforts will be aimed at obtaining more direct experimental evidence for this hypothesis. Recent experimental work showed that, when added to cultures of susceptible cells, interferon binds to cells. This binding probably requires specific receptors on the surface of cells. It is not yet clear whether interferon penetrates across the cell membrane inside the cell and whether such intracellular uptake is required for the biological action of interferon. This question will be studied, along with attempts to characterize the nature and function of cellular receptor sites for interferon.